The earlier study says the drop in temperature, plus fires from the purported impact, wiped out sabertooths, mastodons, and other giant animals, and may have caused the decline of an early civilization known as the Clovis culture.

The 2007 research was based on a combination of archaeological artifacts and extraterrestrial magnetic grains in soil samples found in a thin layer of sediment throughout North America.

The original team, led by Richard Firestone, a nuclear chemist at Lawrence Berkeley National Laboratory, also found what he said are traces of charcoal and microscopic bits of carbon from intense fires ignited by the collision.

However new research, presented at a meeting of the Geological Society of America this week in Portland, Oregon, has taken aim at all of these findings.

Wetlands and Mini-Meteorites

Nicholas Pinter, a geologist at Southern Illinois University, argued that black mats described as charcoal in the 2007 research weren't actually charcoal.

Instead they were from ancient, dark soil formed in a long-ago wetland, Pinter said.

"It's a misunderstanding of what these layers represent."

Likewise, the small amounts of carbon "are not uniquely associated with high-intensity fire," he said.

As for the magnetic grains, they may well have come from outer space, he said.